1. Field of the Invention
The invention relates to a method and an apparatus for producing a thermal transfer print, especially a printing form or a proof, by controlled imagewise heating of a surface layer by means of one or more laser beams and applying the selected area elements to the substrate.
The substrate may be a print material, such as paper, an intermediate carrier to be transferred subsequently to the print material or a printing form, that is to say a printing plate or, in particular, a seamless printing-form cylinder or a seamless printing-form sleeve.
2. Discussion of the Prior Art
Coating a substrate in this way, ill particular by means of a laser, is disclosed by German reference DE 44 30 555 C1. This reference describes a method and an apparatus which enables simple production of a printing form, especially on a seamless printing-form cylinder with a smooth surface, and allows it to be integrated into the printing machine, without the gases which are produced in the laser imaging operation noticeably interfering with the material transfer from the thermal transfer film, that is to say the imaging quality.
A tape-like transfer film having a tape width which is only a fraction of the substrate width is transported between the substrate and the imaging unit, in the direct vicinity of the substrate surface, by means of a tape transport mechanism. The tape transport mechanism, together with the imaging unit and coupled electronically or mechanically, is mounted on a traversing unit, so that the transfer film can be moved over the substrate width uniformly with the movement of the imaging unit.
In conjunction with the laser-induced thermal imaging unit, which is controlled in a known way a by means of a control unit in accordance with an image to be transferred and which, for each image point, introduces heat onto the thermal transfer film, and thus performs a dot-by-dot transfer of the ink-accepting coating of the transfer tape, it is thus possible for the complete substrate, in particular the complete seamless printing-form cylinder, to be imaged all round.
However, at those points which have already been transferred during an imaging operation, the transfer tape can in principle not be reused, analogously to a carbon ribbon in a conventional typewriter. Hence, in the reference cited, the tape can be used only once per section. In order to achieve a number of imaging operations without changing the tape, a very great tape length is therefore necessary. However, the tape length directly influences the diameter of the reel, so that even in the case of a very small number of imaging operations per tape, the reel becomes unfavourably large.
The object of the present invention is therefore to make it possible to carry out a larger number of imaging operations without changing the transfer tape.
Use is made of a transfer tape of a width which is a multiple of the width of the writing tracks of the laser imaging unit. It is thus possible to write a number of tracks alongside one another for various imaging operations. This enables the same section of the transfer tape to be used for a number Of imaging operations. In this case, the position of the point or points of incidence of the laser beam or beams, that is to say the writing track of the laser on the transfer tape, has to be shifted in each case, so that the tracks do not overlap and, an unused track is used for each imaging operation.
It is of course nevertheless possible for the tape lengths to be a multiple of the lengths needed for the imaging operation, so that via a combination of tracks located alongside one another and after one another, a large number of imaging operations per tape becomes possible. The tape is changed only when all the tracks have been used up.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.